In modern control systems, electrically operable valves, which are used as actuators to set at least one parameter and/or a component, are often used. A field of application of valves of this type is the control of wheel brakes, in which the hydraulically or pneumatically generated pressure is controlled in the wheel brakes using the valves. It is problematic in this case that contaminants, which prevent complete closing of a valve, may penetrate in the area of a valve. Leaks may arise in the system due to this. This is not useful in the control of wheel brakes in particular, since the brake pressure in the wheel brakes may no longer be completely maintained and/or the system pressure necessary for brake activation may no longer be provided or may only be provide with more effort.
Electrically operable seat valves which are only partially opened to control the brake pressure in at least one wheel brake are discussed in German Published Patent Application No. 196 54 427. In particular in valves of this type, which are not only completely closed or completely open, but also assume intermediate positions, contaminants may get stuck between the armature and the valve seat. The normally existing filtering system of the pressure medium (e.g., hydraulic liquid) may not be as fine as desired for reasons of flow resistance. As a consequence, the problem of contamination leading to a leak when the valve is not activated or preventing complete closing of the valve when it is activated arises even in valves of this type.
In this method, the current through the valve is regulated using a current regulator. The setpoint of the current regulator is selected by a brake pressure controller using a pulse width modulated signal, for example. In this case, the actual brake pressure is adjusted to the intended brake pressure. For reasons of comfort and control technology, the maximum change of current per unit of time is limited in this case.
Deriving error states, which indicate valve contaminants of this type, in an electrohydraulic brake system on the basis of the behavior of the accumulator pressure is discussed in German Published Patent Application No. 198 07 368.
A method, using the brake pressure in the components which control the wheel brakes is monitored for faults, is discussed in German Published Patent Application No. 198 07 366, using the example of an electrohydraulic braking system. In case of fault, emergency operation is initiated depending on the type of fault, for example, 3-wheel operation or switching off the controller, which leads to uncomfortable braking behavior at the least.